When men first become fathers, their testosterone levels decrease by nearly half, according to a new study.
Annie Engel/Corbis

September 14, 2011

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Dad, it's not just your imagination. Your levels of testosterone — the hormone notoriously responsible for sex drive and other masculine traits — went into the basement after your kids were born. That's one of the findings from a controversial new study out of Northwestern University, which also suggests an evolutionary reason for this plunge in testosterone levels. Here, a guide to this research:

How was this study conducted?Researchers followed a group of more than 600 men from the Philippines for five years; at the start of the study, when the men were 21 years old, none were fathers. The men's testosterone levels, among other biological factors, were measured at the study's onset and then again five years later.

What did researchers discover?First of all, the men with higher levels of testosterone were more likely to become fathers, "possibly because men with higher testosterone were more assertive in competing for women or appeared healthier and more attractive," says Pam Belluck in The New York Times. And while all the subjects experienced a natural drop in testosterone levels after age 21, the decline was steeper for those men who had become fathers.

How does this relate to childrearing?That's the subject of some debate. The study, published in the Proceedings of the National Academy of Sciences, also discovered that men who were most involved with their kids — spending an average of three hours a day or more caring for their children — had the lowest testosterone levels of all. Some experts believe this may be a natural consequence of fatherhood.

What's natural about a plunge in testosterone?One theory: It could be nature's way of ensuring that males commit to parenting. Fathers with lower levels of testosterone might be more willing to help mothers with bathing, feeding, and diaper-changing, thus ensuring that they will raise healthy kids who will survive. Similar hormonal changes are found among new fathers in the animal kingdom. But this hypothesis is meeting resistance, given "the idea that testosterone equals power," says Janice D'Arcy in The Washington Post. In some quarters, "child-rearing could now be considered kryptonite."